Our invention relates generally to filters or resonators, and more specifically to a distributed-constant filter or resonator of the class having a dielectric ceramic body. The device of our invention lends itself to use in communication equipment such as used for car telephone, citizens' radio service, cordless telephone, etc.
Quarter-wave distributed-constant filters find extensive usage in communication equipment. Although a variety of constructions have been suggested for this type of filter, Nishikawa et al. U.S. Pat. No. 4,464,640 represents an example that we believe is most pertinent to our invention. This prior art device is such that a dielectric body of cylindrical shape, with a lead wire extending axially therethrough, is inserted in each of two or more resonance holes formed in a block of dielectric ceramic material. Capacitive coupling is provided between the lead wires and the electroconductive layers on the surfaces of the ceramic block defining the resonance holes.
Basically, we favor this prior art filter because of the simplicity of construction. We do, however, object to the unavoidable fluctuations in the performance characteristics of filters actually manufactured on the fundamental construction described previously. Such fluctuations are unavoidable because the dielectric bodies of plastic or ceramic material are not usually fabricated to very close dimensional tolerances.